FIG. 1 illustrates a typical half-bridge resonant type DC-DC converter 100, which provides a solution for a constant current output power supply. The illustrated converter offers a wide range of output load currents and is stable over the operating range. The converter includes a primary circuit side 102 and a second circuit side 104, which are electrically isolated as described below. The converter includes a first switch 112 and a second switch 114 in a half-bridge switching circuit 110. The switches may be, for example, metal oxide semiconductor field effect transistors (MOSFETs) or bipolar junction transistors (BJTs). In the illustrated embodiment, the two switches are n-channel MOSFETs. The half-bridge switching circuit is connected between a DC input bus 120 (also labeled as VRAIL) and a primary circuit ground reference 122. The drain of the first switch is connected to the DC input bus. The source of the first switch is connected to the drain of the second switch at a common switched node 124 of the half-bridge switching circuit. The source of the second switch is connected to the primary circuit ground reference.
In the illustrated embodiment, the voltage on the DC input bus 120 is provided by a first DC voltage source 130. In the illustrated embodiment, the first DC voltage source is illustrated as a battery; however, it should be understood that the voltage on the DC input bus may be provided by other sources, such as, for example, a power factor correction (PFC) stage, the DC output of a bridge rectifier, or the like, which are supplied from an AC source (not shown). The battery is representative of a variety of voltage sources that provide a substantially constant voltage on the DC input bus.
Each of the first switch 112 and the second switch 114 has a respective control input terminal. In the illustrated embodiment incorporating MOSFETs, the control input terminals are the gates of the two transistors. The control input terminals are driven by a self-oscillating half-bridge gate drive integrated circuit (drive IC) 140, such as, for example, an NCP1392B high-voltage half-bridge driver with inbuilt oscillator, which is commercially available from ON Semiconductor Company of Phoenix, Ariz. The drive IC is powered by a second DC voltage source 142 via a VCC input pin 144. In FIG. 1, the second DC voltage source is illustrated as a battery; however, it should be understood that the second DC voltage source may also be derived from an AC source.
The drive IC 140 is responsive to a timing resistance connected to a timing input terminal (RT) 150 to alternately apply an upper drive voltage on an upper drive terminal (MU) 152 and apply a lower drive voltage to a lower drive terminal (ML) 154. The upper output drive voltage is applied to the control input terminal of the first switch 112. The lower output drive voltage is applied to the control input terminal of the second switch 114. When the resistance applied to the timing input terminal increases, the current flowing out of the timing input terminal decreases, which causes the frequency of the drive voltages applied to the two switches to decrease. When the resistance applied to the timing input terminal decreases, the current flowing out of the timing input terminal increases, which causes the frequency of the drive voltages to increase. The drive IC further includes a brownout (BO) input terminal 160, which is connected to a common node 164 of a voltage divider circuit 162. The voltage divider circuit comprises a first voltage divider resistor 166 connected between the DC input bus 120 and the common node. The voltage divider circuit further comprises a second voltage divider resistor 168 connected between the common node and the primary circuit ground reference 122. The drive IC is responsive to a low voltage on the brownout input terminal to cease switching if the voltage on the DC input bus drops below a selected voltage.
The common switched node 124 of the half-bridge switching circuit 110 is connected to a half-bridge connection terminal (HB) 170 of the drive IC 140. The common switched node is also connected to a first terminal of a resonant inductor 182 in a resonant circuit 180. A second terminal of the resonant inductor is connected to a first terminal of a resonant capacitor 184 at an output node 186 in the resonant circuit. A second terminal of the resonant capacitor is connected to the primary circuit ground reference 122. The resonant inductor and the resonant capacitor are the main resonant components of the resonant circuit, which is driven by the alternatingly connecting the common switched node to the DC bus 120 via the first switch 112 and to the primary circuit ground reference via the second switch 114.
The output node 186 of the resonant circuit 180 is connected to a first terminal of a DC blocking capacitor 190. A second terminal of the DC blocking capacitor is connected to a first terminal 204 of a primary winding 202 of an output isolation transformer 200. A second terminal 206 of the primary winding of the output isolation transformer is connected to the primary circuit ground reference 122. The foregoing components operate as a DC-to-AC inverter produce an AC voltage across the primary winding of the output isolation transformer.
The output isolation transformer 200 includes a first secondary winding 210 and a second secondary winding 212. The two secondary windings are electrically isolated from the primary winding 202. As illustrated, the primary winding is on the primary circuit side 102, and the secondary windings are on the secondary circuit side 104. The two secondary windings have respective first terminals, which are connected at a center tap 218. Respective second terminals 214, 216 of the first and second secondary windings are connected to input terminals of a half-bridge rectifier 220. The half-bridge rectifier comprises a first rectifier diode 222 and a second rectifier diode 224. The second terminal of the first secondary winding is connected to the anode of the first rectifier diode. The second terminal of the second secondary winding is connected to the anode of the second rectifier diode. The cathodes of the two rectifier diodes are connected together at an output node 226 of the half-bridge rectifier. The center tap of the first and second secondary windings is connected to a secondary circuit ground reference 228. In other embodiments having a single, non-center-tapped secondary winding, the half-bridge rectifier with the two rectifier diodes may be replaced with a full-bridge rectifier with four rectifier diodes.
The output node 226 of the half-bridge rectifier 220 is connected to a first terminal of an output filter capacitor 230. A second terminal of the output filter capacitor is connected to the secondary circuit ground reference 228. A load voltage (VLOAD) is developed across the output filter capacitor at the output node of the half-bridge rectifier. The output node of the half-bridge rectifier is also connected to a first terminal of a load 240, which may comprise, for example, one or more light-emitting didoes (LEDs) that emit light when sufficient current passes through the LEDs. A second terminal of the load is connected to a current sensing terminal 242 and to the first terminal of a current sensing resistor 244. A second terminal of the current sensing resistor is connected to the secondary circuit ground reference. When current flows through the load, the same current flows through the current sensing resistor. Accordingly, a voltage develops on the current sensing terminal that has a magnitude with respect to the secondary circuit ground reference that is proportional to the current flowing through the load. In one embodiment, the current sensing resistor has a resistance of, for example, 0.1 ohm such that the effect of the resistance of the current sensing resistor on the load current is insignificant.
When the drive IC 140 operates to apply alternating drive voltages to the first switch 112 and the second switch 114, an AC voltage develops across the resonant capacitor 184. The voltage across the resonant capacitor may include a DC component; however, the DC blocking capacitor 190 transfers only the AC component of the energy stored in the resonant capacitor to the primary winding 202 of the output isolation transformer 200. The transferred energy is magnetically coupled from the primary winding to the electrically isolated first and second secondary windings 210, 212. The first and second rectifier diodes 222, 224 in the half-bridge rectifier 220 rectify the AC energy from the secondary windings into DC energy, which is provided on the output node 226. The DC energy is stored in the output filter capacitor 230 at a voltage determined by the amount of stored energy. Current from the output filter capacitor is provided to the load 240 at a magnitude determined by the voltage on the half-bridge rectifier output node and the resistance of the load.
Because the intensity of the light emitted by the LEDs in the load 240 is dependent on the magnitude of the current flowing through the LEDs, the current is controlled closely. The current sensing resistor 244 senses the current going through the load and develops a voltage VISENSE on the current sensing node 242 proportional to the load current. The voltage representing the sensed current is fed back to a proportional integral (PI) current control loop to provide current regulation. In FIG. 1, the PI current control loop comprises an operational amplifier (OPAMP) 260 having an inverting (−) input terminal, having a non-inverting (+) input terminal, and having an output (OUT) on an output terminal 264. The current sensing node is connected to the inverting input of the operational amplifier via a series resistor 262. A feedback resistor 266 and a feedback capacitor 268 are connected in series between the output terminal of the operational amplifier and the inverting input. A reference voltage (VIREF) having a magnitude corresponding to a reference current (IREF) is connected to the non-inverting input of the operational amplifier. The magnitude of the reference current and thus the magnitude of the reference voltage are selected to produce a desired load current through the load. The reference current may be a fixed reference current to provide a constant load current, or the reference current may be a variable reference current to allow the load current to be varied to thereby change the intensity of the light emitted by the LEDs in the load. The operational amplifier is responsive to the relative magnitudes of the reference voltage VIREF and the sensed voltage VISENSE to provide feedback to the drive IC 140 as described below.
The output 264 of the operational amplifier 260 is connected to the input of a photocoupler 270. The photocoupler (also referred to as an opto-isolator or an optocoupler) has an internal light generation section (e.g., an LED) coupled to the input of the photocoupler. The light generation section is responsive to a voltage applied to the input to generate light. The applied voltage is referenced to the secondary circuit ground reference 228 to which the light generation section is connected. The generated light is propagated internally to the base of a phototransistor in an output section within the same component. The phototransistor is responsive to the generated light to vary the conductivity and thereby to effectively vary the impedance of the phototransistor. The phototransistor has a collector that is connected via a collector resistor 280 to the second DC voltage source 142. The phototransistor has an emitter that is connected to the primary circuit ground reference 122 by an emitter filter capacitor 282. The emitter of the phototransistor is also connected via an emitter resistor 284 to a timing current control node 290. A first timing resistor 292 is connected from the timing input terminal (RT) 150 of the drive IC 140 to the timing current control node. A second timing resistor 294 is connected from the timing current control node to the primary circuit ground reference. As illustrated the photocoupler electrically isolates the secondary circuit voltages and the secondary circuit ground reference in the secondary circuit side 104 from the components in the primary circuit side 102.
When the voltage applied to the input of the photocoupler 270 increases, the effective impedance of the phototransistor in the output section of the photocoupler decreases to raise the voltage on the timing current control node 290. This causes the voltage difference across the first timing resistor 292 to decrease, which decreases the current flowing out of the timing input terminal 150. The decreased current decreases the switching frequency of the drive IC 140.
When the voltage applied to the input of the photocoupler 270 decreases, the effective impedance of the phototransistor in the output section of the photocoupler increases to lower the voltage on the timing current control node 290. This causes the voltage difference across the first timing resistor 292 to increase, which increases the current flowing out of the timing input terminal. The increased current increases the switching frequency of the drive IC 140.
The illustrated drive IC 140 has a fixed deadtime between turning off one of the switched outputs and turning on the other of the switched outputs. The fixed deadtime causes the duty cycle of the on-time of each of the first and second switches 112, 114 to decrease with increased frequency and to increase with decreased frequency. A decrease in duty cycle causes the energy transferred to the load to decrease. An increase in duty cycle causes the energy transferred to the load to increase. Thus, the load current decreases with increased switching frequency, and the load current increases with decreased switching frequency.
From the foregoing, it can be seen that when the current through the current sensing resistor 244 generates a voltage VISENSE that is less than the voltage VIREF corresponding to the reference current IREF, the output voltage of the operational amplifier 260 increases. The increased output voltage produced by the operational amplifier causes the photocoupler 270 to increase the light generated between the input section and the output section, which causes the photoresistor in the output section to increase conductivity and thus decrease the effective impedance. This causes the voltage on the timing current control node 290 to increase, which decreases the current flowing out of the timing input terminal 150. The decreased current decreases the switching frequency of the drive IC 140, which increases the duty cycle of each switching voltage applied to the respective control input terminals of the first switch 112 and the second switch 114. The increased duty cycle has the effect of increasing the energy transferred to the output filter capacitor 230, which increases the voltage on the output node 226, which increases the current flowing through the load.
When the current flowing through the load is greater than the reference current, the opposite transitions occur. The voltage on the output of the operational amplifier 260 decreases. The effective impedance of the output section of the photocoupler 270 increases to cause the voltage on the timing control node 290 to decrease. The current flowing out of the timing input terminal 150 increases. The switching frequency of the drive IC 140 increases, which decreases the duty cycles of the two switch control voltages. The energy transferred to the output filter capacitor 230 decreases, which decreases the voltage on the output node 226, which decreases the current flowing through the load 240.
As shown above, the output isolation transformer 200 and the photocoupler 270 between the primary side circuit 102 and the secondary side circuit 104 effectively isolate the voltages and the ground references on the two circuits. However, many components are required to realize output current control for an isolated power supply. The additional components required to perform secondary side sensing and to feed back the sensed values to the primary side controllers increase the parts costs and also complicate the layout of a printed circuit board (PCB) onto which the components are mounted. Accordingly, a system that accurately senses the load current on the primary side would simplify the circuit design and PCB layout for an isolated power supply.